The invention relates to a beam-shaping optical element having an entrance surface, an exit surface located opposite thereto and an optical axis which coincides with the Z axis of a three-axis rectangular XYZ system of coordinates for converting a beam having a first ratio between a first angular aperture in the YZ plane of the system of coordinates and a second, smaller angular aperture in the XZ plane into a beam having a second, smaller ratio between said angular apertures, said element realising different angular magnifications in said two planes.
The invention also relates to a radiation source unit and to an optical scanning unit, both including such a beam-shaping element.
A beam-shaping element of this type is known, for example in the form of a prism, a cylindrical lens or, as described in European Patent Application no. 0 286 368, a single lens element whose entrance and exit surfaces have a toroidal shape. This beam-shaping element is generally used in combination with a diode laser which emits a beam whose angular aperture in a plane parallel to its active layer, known as the lateral plane, is smaller than the angular aperture in the plane perpendicular to the active layer, known as the transversal plane. In the field known as the far field, the beam of such a diode laser has an elliptical cross-section. In a device in which such a diode laser is used as a radiation source, for example, a reading and/or writing device for optical record carriers in which an audio or video program or data are or can be stored, or a printer, a round and small, preferably diffraction-limited radiation spot must be formed on the medium to be scanned. To this end an imaging system, or objective system, by means of which the radiation spot is formed must be filled with a radiation beam having a circular cross-section. It is known that, starting from a diode laser, such a beam can be obtained by arranging a beam-shaping element between this laser and the objective system and at some distance from the diode laser.
In known systems using a beam-shaping element, stringent requirements must be imposed on the beam-shaping element as well as on the positioning of this element with respect to the radiation source. The known beam-shaping elements are designed in such a way that the beam shaping, hence the magnification or reduction of the beam cross-section, is realised in only one of the planes, the transversal or the lateral plane. Since the shaping in this plane must be relatively strong, stringent requirements are imposed on the parameters playing a role in beam shaping.
Moreover, in known systems using beam shaping, the beam-shaping element is arranged at a relatively large distance from the radiation source, viz. where the diverging beam emitted by the source has the required cross-section. However, stringent requirements are also imposed on the axial or Z position of the exit plane of the diode laser with respect to the beam-shaping element. If the Z position of the diode laser exit plane differs from the desired position, the laser beam will have a wavefront with a quadratic defocusing term at the area of the entrance surface of the beam-shaping element. The quadratic distortion which is a function of the angle at which a given portion of the wavefront is viewed from the centre of the radiation source is transformed in different manners by the beam-shaping element in the two main cross-sectional planes, the XZ plane and the YZ plane. In fact, the known beam-shaping elements have a relatively large angular magnification factor or scaling factor in one of these planes and a magnification equal to one in the other plane. If the beam-shaping ratio is larger than, for example two, the defocusing of the radiation source is substantially completely transformed by the beam-shaping element into a defocusing of the beam in only one of the main cross-sectional planes. This means that the beam emerging from the beam-shaping element has become astigmatic. Whereas in the optical systems under consideration a defocusing of the radiation source itself can be corrected by an active focus control for the objective system, an astigmatic wavefront error can no longer be eliminated. Consequently, stringent tolerance requirements are imposed on astigmatism. If an average wavefront deviation, i.e. the square root of the integral, across the surface of the wavefront, of the square value of the wavefront deviation divided by the surface, indicated by OPD.sub.rms, of 0.02.times.the wavelength (.lambda.) is still allowed, the astigmatic wavefront error W.sub.A should be smaller than 0.1 .lambda.. This means that the defocusing .DELTA.Z of the radiation source with respect to the beam-shaping element defined by ##EQU1## may at most be of the order of 1.5 .mu.m if the numerical aperture NA of the beam-shaping element is 0.35 and .lambda.=0.8 .mu.m.
In the commonly used optical systems, in which the beam-shaping element is arranged at a relatively large distance from the diode laser, such a strict tolerance requirement is difficult to satisfy. Due to temperature variations and mechanical shocks, axial displacements amounting to many microns may occur between the diode laser and the beam-shaping element.